1. Field of the Invention
This invention relates to the transmission of information signals from a borehole location, and more particularly to a novel amplifier and drillstring transducer for transmitting low-frequency-electromagnetic information signals as part of a drillstring/ earth telemetry (D-S/ET) measurement-while-drilling (MWD) operation.
2. Description of the Prior Art
Modern drilling techniques for oil wells and the like require near-real-time transmission from the downhole location near the end of the drillstring up to the surface. Various sensory devices are provided in the drillstring so that information on downhole temperature, the drilling medium, drillstring orientation, etc. can be measured and transmitted to the surface. In order to optimally control the drilling process and achieve economic drilling of an oil well or the like, this information should be provided while the drilling is going on, a mode referred to as measurement-while-drilling (MWD).
The essential element in borehole electromagnetic telemetry is the wave-propagation means between the downhole and uphole terminals, including special provisions for coupling the signals into and out of the propagation medium at both ends with mode couplers, consistent with the specific electromagnetic and geometric contraints at each terminal. One particular electromagnetic propagation method is sometimes referred to as the drill-string/earth telemetry (D-S/ET) mode, in that in some ways it behaves like a two-wire transmission line. In this mode, the drillstring is one conductor and the earth's bulk conductivity is the other. The loss mechanisms include (a) transducer losses at each terminal, (b) mismatch losses at each terminal, (c) series (I.sup.2 R) losses associated with the "conductors", and (d) shunt (V.sup.2 G) losses associated withthe shunt-path conductivity between the "conductors".
In general, in D-S/ET the propagation path is principally characterized by increasing attenuation (loss of signal) with increasing distance (depth), increasing data rate, and increasing conductivity of the earth's bulk. Provided that a carrier frequency is used which provides skin depths much greater than drillstring diameters, then factors such as drillstring diameter, wall thickness, material, and joints become second-order along with the electrical characteristics of the drill fluid so long as the formation is reasonably tight and/or under positive pressure.
In order to transmit an information signal by the D-S/ET method up through the earth from a downhole location, one or more electrical discontinuities must exist in the drillstring at the point from which the information is to be transmitted. Two methods that have been used for D-S/ET transmission are direct coupling and toroidal coupling. The direct coupling method requires a complete electrical discontinuity in the drillstring so that a potential difference can be produced across adjacent conducting faces of the drillstring. The toroidal coupling technique, which is more conventional, requires an electrical discontinuity only in the outer portion or sheath of the drillstring, to prevent the existence of an unwanted short-circuited turn.
The conventional way of implementing the toroidal coupling technique has been to configure the drillstring transducer apparatus as a slender toroidal transformer with a mandrel of conducting, strengthening members running through the center of the toroidal core, the mandrel serving as both the principal structural element and as one-half of a one-turn secondary winding. The toroidal transformer provides impedance matching between the low frequency information signals, which may be less than 10 Hertz, and the very-low-impedance earth-load through which they are transmitted, which may be as low as 50 millohm. In the case of a direct coupled system, in which a complete electrical discontinuity is provided in the drillstring, a separate multi-turn secondary transformer may be used with or without an electrically-conducting mandrel.
Because of the relatively high degree of impedance matching required, the toroidal transformers used in the prior art have been quite long, typically extending for ten to thirty feet along the drillstring. A steel sheath has been used to protect the core and windings, the sheath providing structural bending strength but little significant tensional or torsional strength. The internal mandrel provides tensile and torsional strength, but less than that provided by the rest of the drillstring. The low frequency information signals required a large transformer core volume, the volume of the transformer being inversely proportional to the frequency raised to the 3/2 power.
After the sensor signals have been conditioned and their information modulated onto a carrier signal, the modulated signal has to be amplified before it can be transmitted. This has been done in the past by the use of conventional amplifiers operating at the carrier-signal frequencies. This combination of signals, conventional amplifiers and large impedance-matching transformers exhibit a number of inherent disadvantages. The equipment is large and expensive to build, and is fairly low in strength because the shape must be accomodated to the narrow drillstring. The transformer is restricted to a single secondary turn, making it difficult to adjust the turns ratio when necessary to achieve efficient impedance matching. The power handling capability is restricted due to the limited amount of space for the magnetic core material. Additionally, the mandrel restricts the flow of drill fluid through the interior of the drillstring.